Electronic devices, such as transistors, diodes, resistors, capacitors, transducers, monolithic packages, hybrid microcircuits, contain one or several dies or chips bonded onto a rigid substrate with an adhesive.
The die is a semiconductor on which all the active and passive microcircuit elements have been fabricated using one or all of the techniques of diffusion, passivation, masking, photoresist and epitaxial growth.
The substrate is the material upon which circuits are printed or fabricated. It is used as a mechanical support but may serve useful thermal and electrical functions. It may be made of a number of ceramic materials, such as alumina, barium titanate, beryllia, boron nitride, porcelain, multilayer ceramic, aluminum nitride, silicon carbide, and other materials such as steatite, glass, sapphire, metal (such as steel) enameled with glass, quartz, epoxy, polyimide, etc.
The substrate, in turn, is bonded to a larger, rigid plate, board package or case, herein referred to as support. It is usually made of metal, ceramic, glass or high temperature plastic, providing protection, mechanical support or hermeticity.
Substrates are usually fixed to the support with an epoxy adhesive. It is currently available as a B-stage epoxy sheet coated on both sides of a glass fabric for support. The epoxy adhesive presents a great number of disadvantages:
Since the epoxy is of a tacky, semi-solid consistency, it must have a glass fabric carrier in the middle and a release-coated plastic film on both sides of the sheet;
It requires freezing or refrigeration for storage;
It has to be cured or polymerized for 1-24 hours at 90.degree.- 200.degree. C.;
It usually contains large quantities of ionic impurities, often up to 400 ppm Cl.sup.- and Na.sup.+, which are conducive to corrosivity and hence decreased reliability of the circuit;
Once the substrate is bonded with the epoxy thermosetting adhesive, it is very difficult to remove it if rework is required for some reason.
The object of this invention is to overcome these difficulties.